fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-JINJA2-9292516

Author: snyk-bot

requirements.txt

@@ -15,7 +15,7 @@ flask-sqlalchemy==2.4.4
 flask==1.1.2
 importlib-metadata==3.1.1; python_version < '3.8'
 itsdangerous==1.1.0
-jinja2==2.11.2
+jinja2==3.1.6
 jsonschema==3.2.0
 markupsafe==1.1.1
 marshmallow-sqlalchemy==0.24.1